Conjugation in the yeast Saccharomyces cerevisiae between haploid cells of opposite mating types a approximately and alpha is mediated in part through two diffusible peptide mating hormones (a approximately-factor and alpha-factor), which have been purified to homogeneity. Haploid cells incubated with the hormone from the opposite mating type (or cells in mating mixtures) exhibit several characteristics: (1) they arrest in the G1 stage of the cell cycle; (2) they became (more) agglutinable; (3) they synthesize a somewhat different cell wall that is localized on the surface; (4) they elongate to pear-shaped forms. In addition, we have shown that a approximately cells secrete more a approximately-factor activity when grown with alpha cells (possibly through the action of alpha-factor). We will attempt to elucidate the mechanism of hormone response in the following studies: a. The isolation and characterization of alpha mutants that show no or only partial response to a approximately-factor; b. The nature of the increase in a approximately-factor activity in mixed a approximately and alpha cultures, whether by secretion, synthesis or activity. c. The location of the primary receptors for the mating hormones. d. The identification of the TUP1 gene product ( alpha tup1 strains exhibit partial hormone response constitutively).